Iron oxide pigments which contain iron in the +2 oxidation state are thermodynamically unstable compared to iron(III)oxide (Fe.sub.2 O.sub.3). They may be partially or completely oxidised in the presence of air or oxygen, for example: EQU 2Fe.sub.3 O.sub.4 +1/2O.sub.2 .fwdarw.3Fe.sub.2 O.sub.3 ( 1).
Reactions of this type are known, for example in the case of black iron oxide pigments which correspond in composition and structure to magnetite. As a result of oxidation, the pigment loses its most important property, namely its color, and is thus unuseable. The tendency towards oxidation understandably increases with increasingly fine particle size of the pigments and thus increasing specific surface area thereof.
The same also applies to mixtures of black iron oxide with other iron oxide coloring pigments, e.g. red iron oxide or yellow iron oxide, as they are produced for shades of brown.
If, in the case of coloring pigments, it is the loss of the color properties which renders them unuseable, then, in the case of magnetic pigments containing iron(II), it is the loss of the magnetic properties, which is also caused by oxidation. Above all, finely-divided magnetite pigments and mixed phases between magnetite (Fe.sub.3 O.sub.4) and maghemite (.gamma.-Fe.sub.2 O.sub.3) having a high iron(II) content are at risk. However, the following are also sensitive to oxidation: mixed phases of magnetite with maghemite and/or ferrites, for example cobalt ferrite, and those magnetic pigments which are composed of a core of Fe.sub.3 O.sub.4 or an iron oxide having an oxidation state between Fe.sub.3 O.sub.4 and .gamma.-Fe.sub.2 O.sub.3, and a coating surrounding this core consisting of magnetic metal oxides, in particular of iron and cobalt. In addition to the term "mixed phase", the technical literature also provides the expression "berthollides" for the above described compositions.
Attempts have been made to reduce the oxidation sensitivity of finely-divided ferrimagnetic magnetite pigments by a treatment with heterocyclic organic compounds (German Offenlegungsschrift No. 2,744,598). Compared to untreated pigments, the treatment has provided a considerable improvement which, however, proved impossible to increase above a certain level. Moreover, the heterocycles used are only physically adsorbed on the pigment and thus pass into the water-soluble constituents to a considerable extent. Consequently, incompatibilities in different binder systems may result.
German Offenlegungsschrift No. 2,743,298 discloses metal particles which consist essentially of iron and contain boron on the surface in the form of borate. European Patent No. 3100 describes a ferrimagnetic iron oxide which may also contain boron in addition to other elements for protection against sintering during conversion processes.
However, no suggestion of thermostabilisation of iron oxide pigments containing iron(II) may be inferred from these references.